This invention relates to an apparatus and method for providing active vibration isolation at an engine mount to prevent engine vibration from propagating from an aircraft engine into the wings and fuselage of an aircraft.
In aircraft cabins, tonal noise is generated by the fundamental frequencies of engine rotations. The generation of this noise has been an issue for many years. The tonal noise is caused by residual imbalances in the engines rotating parts, such as low and high speed turbines, fan blades, compressors, etc. Even though the imbalance from rotating parts, including shafts and blades, are closely controlled in manufacturing, due to improved manufacturing methods, the imbalance can still develop due to changes in operation, or through deterioration of the system over time. As a result, significant “once-per-revolution” vibration excitations from the rotating components are introduced into engine operations.
It is these kinds of vibrations which propagate through wing and/or fuselage structure and produce annoying low frequency tonal noise in the aircraft cabin.
This tonal noise is usually a major contributor to the overall cabin noise level. According to many noise evaluation standards, additional penalties will be applied to the overall noise level if significant tonal noise exists. These tones are usually in a low frequency range. For example, engines powering mid to larger commercial aircraft usually have less than 100 Hz low pressure (LP) system 1/rev frequency and less than 200 Hz high pressure (HP) system 1/rev frequency. For small aircraft such as regional jets, the LP system 1/rev is around 100 Hz, and the HP system 1/rev is about 300 Hz. Psychoacoustics analysis indicates that an individual can be easily fatigued if exposed to low frequency noises, especially with long time exposure, such as in the long range air travel.
It is understood that these tonal vibrations cannot be avoided. In engine manufacturing, the rotating components are balanced carefully. However, during the operation, the balance can change, introducing an imbalance into the structure. The system deterioration with service time can also introduce imbalance. The imbalance-induced vibrations transmit through the engine mount, wing structures, fuselage structures, and finally excite the cabin interior structures, such as trim panels. The vibration of the interior structure propagates the noise into the cabin.
Traditionally, “soft” (i.e. flexible or shock absorbing) engine mounts have been the least expensive and most effective way to reduce the vibration transmission. However, for large commercial aircraft, the engine vibration frequency can be as low as 45 Hz, which means that the soft mount isolator needs to be designed to have resonance much less than 45 Hz. Such a “soft” mount design results in a large displacement during the engine speed up, which is undesirable and air frame manufacturers wish to avoid. Further, the reliability and durability of soft engine mounts is an issue, as their reliability and durability are less than hard engine mounts.
Therefore, there still exists a need to reduce tonal noise generation in applications where a hard engine mount is used.